PROJECT SUMMARY/ABSTRACT The retina generates rod and cone photoreceptors with distinct light sensitivity and synaptic connectivity to execute night- and day-time vision, respectively. Disruption of particular transcription factors affect photoreceptor differentiation during development and the maintenance of photoreceptor-specific properties in the adult. For instance, loss of Neural Retina Leucine zipper (NRL) or Nuclear Receptor Subfamily 2 Group E Member 3 (NR2E3) in mouse retina transform cells with rod-fate into cone-like cells, at embryonic or adult stages. Rod->cone transformations have been found to protect photoreceptors from degeneration in mice with certain retinal diseases. However, it is unclear whether and how transformed photoreceptors wire with their downstream target neurons, particularly bipolar cells (BCs), to convey visual signals. Rod->cone transformations in some retinal diseases can, however, compromise visual function. The possibilities of reversing such photoreceptor transformations, restoring proper connectivity and re-establishing normal visual function are unknown. To advance knowledge in these areas, this proposal will determine the connectivity patterns of BCs with transformed photoreceptors under different conditions. A combination of immunohistochemistry, electron microscopy, and electrophysiology will be used to examine mice with photoreceptor transformation at different ages and to different extents. In Aim 1, mice with Nrl deletion at embryonic (Nrl-/-) and adult (NrlCKO) stages will be used to determine the connectivity between BCs and cones or cone-rod hybrids derived from rod-fated cells at different ages. In Aim 2, rd7 mice, which model human retinal disease with mutations in Nr2e3, will be used to further assess BC wiring with cones and cone-rod hybrids transformed from rod precursors. Restoration of normal rod/cone ratios in adult rd7 will be attempted, using adeno-associated viruses (AAVs) to drive NR2E3 expression in transformed photoreceptors. The results will indicate the possibility of rod restoration in adult rd7, as well as the plasticity of the mature retina in recovering disrupted circuitry. In both aims, the visual responses of BCs or ganglion cells will be investigated to assess the impact on visual function by photoreceptor transformation and restoration. Overall, the knowledge gained from this proposed study will not only advance our understanding of the cellular mechanisms underlying photoreceptor type-specific connectivity, but also provide insights into designing future retinal repair strategies. The applicant will receive training from Drs. Rachel Wong (primary mentor), Thomas Reh and Fred Rieke at the University of Washington to attain the essential knowledge, techniques and professional skills to pursue an independent research career dedicated to discovering the basic mechanisms that regulate neural circuitry assembly and repair.